<template>
  <ul class="list">
    <li 
      class="item" 
      v-for="item of letters" 
      :key="item"
      :ref="item"
      @touchstart.prevent="handleTouchStart"
      @touchmove="handleTouchMove"
      @touchend="handleTouchEnd"
      @click="handleLetterClick">{{item}}</li>
  </ul>
</template>

<script>
export default{
  name: 'CityAlphabet',
  props: {
    cities: Object
  },
  computed: {
     letters () {
       const letters = []
       for(let i in this.cities){
        letters.push(i)
       }
       return letters
     }
  },
  data () {
     return {
       touchStatus: false,
       startY: 0,
       timer: null
     }
  },
  updated () {
     this.startY = this.$refs['A'][0].offsetTop
  },
  methods: {
    handleLetterClick (e) {
     this.$emit('change',e.target.innerText)
    },
    handleTouchStart () {
      this.touchStatus = true
    },
    handleTouchMove (e) {
      if(this.touchStatus) {
        if(this.timer) {
         clearTimeout(this.timer)
      }
      this.timer = setTimeout(() => {
         const touchY = e.touches[0].clientY - 79
         const index = Math.floor((touchY - this.startY) /20)
         if(index >= 0 && index < this.letters.length ) { 
         this.$emit('change', this.letters[index])
       }
      }, 16)
    }
  },
    handleTouchEnd () {
      this.touchStatus = false
    }
  }
}
</script>

<style lang="stylus" scoped>
  @import '~styles/varibles.styl'
  .list
    display: flex
    flex-direction: column
    justify-content: center
    position: absolute
    top: 1.58rem
    right: 0
    bottom: 0
    width: .4rem
    .item
      line-height: .4rem
      text-align: center
      color: $bgColor
  
</style>